1. Field of the Invention
The present invention relates to harmonic frequency filtering circuits, and, more particularly, to harmonic frequency filtering circuits monolithically realizable on a semiconductor substrate.
2. Description of the Related Art
Solid-state power amplifiers are frequently required to meet both high efficiency and low harmonics requirements simultaneously. In order to achieve high efficiency, such amplifiers are often designed to operate in nonlinear modes, such as class-B or -C, in which the output stage must be driven into compression. In such nonlinear modes, the active device (for example, a transistor) generates excessive levels of harmonics which are undesirable and must be filtered to improve the signal-to-noise ratio of the device. It has been shown (for example, D. M. Snider, "A Theoretical Analysis and Experimental Confirmation of the Optimally Loaded and Overdriven RF Power Amplifier", IEEE Transactions on Electron Devices, vol. ED-14, No. 12, Dec. 1967, pp. 851-857) that by terminating the even-order harmonics of the amplifier into a short-circuit (low impedance) while terminating the odd-order harmonics into an open-circuit (high impedance), one can achieve a high efficiency mode of operation (class-F) while at the same time filtering the unwanted harmonics from the output of the amplifier. In particular, the short-circuit termination of the second harmonic is important since it is usually the dominant harmonic.
Present commercial power amplifiers (such as those used in portable communication systems) often use hybrid thick-film circuit approaches to properly terminate the unwanted harmonics and achieve high efficiency modes of operation.
FIGS. 1a & 1b show schematic views of two conventional thick-film circuit approaches to harmonic termination of power amplifiers. The bipolar transistor is shown by way of example and the approaches are valid for other types of circuits.
In the approach of FIG. 1a, an AC grounded (by means of the bypass capacitor 60) quarter-wavelength (at the fundamental frequency) transmission line stub 62 shunts the output of the amplifier 100. At the fundamental frequency and its odd-order harmonics, the transmission line transforms the low impedance (short) of the capacitor 60 to a high impedance (open) at node 64. At even-order harmonics, however, the desired low impedance of capacitor 60 is presented to the output of the amplifier 100 at node 64.
FIG. 1b shows another thick-film circuit approach in which a parallel-resonant tank 70 is placed at a quarter-wavelength (at a quarter-wavelength (at the fundamental frequency) distance from the amplifier 100 output. The values of inductor 72 and capacitor 74, which constitute the parallel-resonant tank 70, are chosen to produce a resonant frequency that coincides with the fundamental frequency of the amplifier 100 according to the following relationship: ##EQU1## Since the tank circuit 70 is an open circuit at the resonant frequency, the fundamental RF frequency of amplifier 100 is unaffected by it. At harmonic frequencies, however, the tank circuit 70 is a low impedance capacitive termination. This low impedance is transformed by the quarter-wavelength transmission line 76 and is seen by amplifier 100, at node 78, as a low impedance at even-harmonic frequencies and as a high impedance at odd-harmonic frequencies.
The approaches illustrated in FIG. 1a & 1b are not applicable to monolithic integrated circuit technology, especially at UHF and low microwave frequencies, due to the large dimensions required for the quarter-wavelength transmission lines. Therefore, it can be seen that there is a need for a method for removing undesired harmonic signals from a circuit output that is compatible with monolithic integration.